hum in high power tweed twin amp

[stringbender.dk]

Hi!

I got a high power tweed twin amp that I bought from Tube amp doctor.
it makes a hum noise, and I have tried to reduce it with more aluminium tape around the chassis, better grounding, but with no succes.

I got it to a tech, and he  put a trimpot in it, he changed the two resistors that is mounted on the bulk lamp, and some other stuff. but with no luck.

he wrote to me that if he pull out the v3, the noise/hum stop. but he can't find the reason of what generates the hum....
he will try again tomorrow.

But meanwhile, do you guys have any advise on what could be the problem?

THanks!

[stringbender.dk]

the tech told me today that he noticed when he switch the standby off, then the hum/noise are going on for about two seconds.

does that say you something?

[pompeiisneaks]

Troubleshooting this type of thing requires a few things at a minimum:

1. a schematic of the exact amp
2. photos of the amps inside to see if we see obvious issues
3. how old the amp is itself. 

also if the amp tech can't solve it, either you need to find a different amp tech, because even if we find general areas to 'try' you're going to have to contact the amp tech back and forth, etc, which will make it a lot harder. 

If you can't work on an amp yourself, having us go through you to your amp tech is going to make it 10X harder to troubleshoot. 

that being said, Noise in amps can be caused by tons of things.  Is this a 120Hz hum or 60Hz hum? 

If 60Hz then the heaters are getting too close to the signal path somewhere and that needs to be sorted, or there is some other serious problem with the heaters.
if 120Hz then the power filtering has issues.  If it is an old enough amp, it may need the electrolytic capacitors swapped out for new ones.

Tubes may have gone bad, and need replacing.

All things the amp tech should know to check.

~Phil

[DummyLoad]

i think he's in denmark; so that would be 50Hz/100Hz hum/buzz.


--pete

[Leevi]

Is it ground hum i.e. the hum is coming on zero-volume?


Does the hum increase with volume?


There can be many reasons ĺike:
- bad or wrong grounding
- bad filter caps
- ripple on bias voltage source
- wiring
- heavily unbalanced power tubes
- e.t.c.


Analyze first if it is so celled ground loop hum (exists with zero-volume)


/Leevi

[stringbender.dk]

the hum is there even with the volume on zero.

A man I thought was a tech build the kit for me about 10 years ago.   
But it was a bit noisy, so after using it for a while, it ended up in a corner at home for several years.
Then for about two month ago, I watched a video on youtube with a guy who had build a amp like mine. it sounded awesome. like mine, but with no noise...
So I took the backpanel off, and tried to look at the layout and how my amp was putted together.
I realised that it was not build correctly. some grounds wasn't where they supposed to be. the heaters wasn't wrapped together, and was soldered very short and straight to the tubesockets, the mainwire wasn't wrapped, no wire was wrapped at all... and some places he had took of the shielding on the wire. the filter caps turretboard was loose in its cabinet. I fixed that... 

I fixed those things and took it to a tech at a musicstore.  and that's were we are now.  the hum is still there. it stops when the V3 is pulled out.
he has tried several tubes in V3 with no succes. here in Denmark we use 230volt. I wish I could put in some pictures, but I don't know how

[shooter]

when you hit reply, below the text you type is a click;
+ Attachments and other options

use that with browse to find image file, click image file you want, walla.  then click post

(there are file type n size restrictions listed)

[pompeiisneaks]

What video were you watching?  Also, do you have photo's of the inside of the amp?  An exact schematic and/or layout that was used to build it?

If we don't have those, it becomes pretty difficult to troubleshoot blind.

~Phil

[stringbender.dk]

Her is how the amp looked when I took the backpanel off

[stringbender.dk]

This was the video I watched:

I know he did some mods to it.  he installed a mastervolume and pulled out two output tubes and wired the speakers different.

This is the papers that followed the amp:'
http://www.tubeampdoctor.com/images/artikel/Full_documentation_Two-Twelve-65.pdf

[pompeiisneaks]

At quick glance it's looking 'fine' but it's pretty dark, also showing close ups of the different tube sockets will make it easier to see if something in the heater wiring is the problem.  So this is a Mojotone high powered deluxe then?   Does this look like the right schematic and layout?  http://www.mojotone.com/kits/TweedAmpKits_x/Mojotone-Tweed-Twin-High-Power-Style-Amp-Kit

Just wanting to make sure we're making apples to apples comparisons.

~Phil

[pompeiisneaks]

In the photo, it's hard to see for sure, but I don't know that I see two 100ohm resistors to ground from the pilot light assembly?  Are they there?  If not the heaters will be a lot noisier.  make sure you get two to ground from each side of the heaters.

~Phil

[Leevi]

I don't see a choke in the picture?


/Leevi

[drew]

If the "tech" cannot offer anything beyond reporting that the hum stops if he removes one of the tubes, you need to find a real tech.

[stringbender.dk]

I send you a link together with the video. but if you didn't saw it, it is here. this is the papers that followed the amp kit
http://www.tubeampdoctor.com/images/artikel/Full_documentation_Two-Twelve-65.pdf

Phil, there is two resistors om the lamp and they are soldered together to the ground.

Leevi: choke?   do you mean the fuse?

the tech is a real tech. he has his workshop in the local musicstore. and he make a lot of other reperations.  but my amp must be doomed, or something....  :)

[pompeiisneaks]

Oh sorry didn't see that.  Yeah that's looking pretty similar to what I can see on the amp.  Honestly, it looks fine, not the best nor the worst build.  The hum could be bad heater clearance, or it could be bad solder connections on the power filtering capacitors so they're not working very effectively.  I'd get a wooden chopstick, and with power on, try tapping on the different capacitors, connections etc, see if slightly moving anything around makes the noise worse or better, etc.  Just be VERY careful and don't put your hand inside the amp with it on or it can kill you.  If you find something that makes things worse/better, it may either be a bad component or a bad solder connection.   As mentioned, we can't easily be the third party in troubleshooting something like this, as we'd need you to do things you may not really understand, like make sure you have continuity between the right points on the amp, etc. 

~Phil

[Leevi]

Leevi: choke?   do you mean the fuse?

No, I mean this ("drossla")




https://en.wikipedia.org/wiki/Choke_(electronics)



Choke can decrease or even remove a certain type of hum. Another alternative is
to add an extra filter stage and power resistor before the 1st filter stage.


/Leevi

[stringbender.dk]

I was half sleeping when I wrote the last comment....   
I know what a choke is....  sorry   

Phil I understand that.   The amp is at the tech now.   we write mails to each other. he write what he has been trying, I write what I have found on the internet, or what you guys have been giving of good advises 

[drew]

the tech is a real tech. he has his workshop in the local musicstore. and he make a lot of other reperations.  but my amp must be doomed, or something....  :)

Your amp is not doomed. This tech is not being asked to figure out how to operate a spacecraft left on Earth by ancient aliens and there's no instruction manual in the glove compartment. He's being asked to diagnose what is almost certainly a prosaic wiring error, poor connection or component fault in a popular, thoroughly understood and documented, 50-60 year old tube amp design. (Whatever minor tweaks the kit supplier may have made to Fender's original layout should not make any difference.) If he can't find the problem in a couple of hours, and he's flailing around with adding trim pots and asking you to look up stuff on the internet, I have to think that... wait for it... there is something rotten in the state of Denmark!  

[Ed_Chambley]

Looks like wiring or what is lead dress.
Look at your power tubes green heater wires and notice a blue wire against it. The blue wire is probably your output transformer primary.
I can see screen wires parallel with plate wires.


Looking at the preamp tube sockets from the photo makes them appear bent, solder blobs and generally nothing separated.


Not saying a choke wouldn't help, but if this noise is real loud, get a wooden stick like a chopstick and turn the amp on with the back chassis open. Push the wires and parts around some and see if anything changes when you do.  Be careful use one hand.


TAD is not known for poor products. It is not the amp, it was the amps builder.

[bnwitt]

(4) 6L6s and no choke in the power supply?  The schematic is definitely showing one.

[92Volts]

Could be a bad tube. A heater-to-cathode short is just one example of a tube fault that will cause (tons of) hum. Has the tech tried switching the V3 tube  with other positions or a known-good spare?

If running without V3 gets rid of the hum it's either being introduced earlier in the circuit and V3 amplifies it, or it's produced in V3's area of the circuit and/or the tube itself.

Does this tech own an oscilloscope?

[bnwitt]

Boy I tell you one thing that drives me nuts on some amps is too many wires are yellow   I do see the wires from the choke at the tagboard.  I don't see the two 100 ohm resistors that create the artificial center tap for the filament secondary though.  Maybe I just can't see them.  Ok I see someone else asked about that and they are there.  I have to say lead dress is very bad especially the heater wiring.

[mresistor]

It looks as though there may be a green wire with red stripe going to a PT bolt upper left  - really hard to tell - but would explain the lack of artificial center tap on the filament line.

[mresistor]

The cap on the standby switch would be to prevent popping when engaging the switch.  The yellow leads from the pwr lamp look to be going straight to the second power tube filament wire connections pins 2 & 7.  There is barely any twist to the filament wires or AC wires to counteract hum. Lead dress isn't very good. It looks like a bowl of spaghetti noodles.

[mresistor]

Stringbender take a look at this low power tweed twins lead dress..  much more logical and neat..  and notice how the filament wires and AC wires are twisted neatly to prevent hum.

[stringbender.dk]

Thank you all very much for youre continuing help and advises  :)

After I took this picture of my amp, I cleared up the mess of all that spaghetti :)   I wish I took a picture after that.... 

I placed the ground wires where they should be.    I took the yellow leads away from the power lamp and tube 2 socket.   I twisted all the leads to the rectifier. I twisted the green wire from the transformator to the output tube 1, I connected the output tube 1 with the power lamp. I know that it is common to connect the green wire to the power lamp first, and then down to the tube, but the man who build it, had cut the wire, so I had to do it this way....
Then I ordered som new heater wire and replaced the heaters that is on the picture.      There was some wiring from the turetboard with no cloth on. I replaced them with a new clothwire.

Then I took it to the tech. I did not turn it on after I did this, because I wanted him to check it first.    When he did that, he said that it still was noisy. And he found out that it dissapered when he pull out the V3.   

he installed a trimpot with no luck. He tried another tube with no luck. he will try some others next week.

I have write some of your comments and advise to him.  I have send a link to robrobinette, where he mentioned some things about the heaters. some out of phase, and some in phase...
We have discussed the "dead cap",  I noticed that on the new layout on tube amp doctor the two resistors on the power lamp is soldered to the "bridge" where some of the tiny leads from the transformator also is soldered to.  In my amp, they are soldered to the chassis. (ground)   I wrote that to him, and some other stuff...   So I'll hear from him tomorrow again, when he has spent some time on it again

[mresistor]

It really doesn't matter if you take the 6.3 vac to the pwr lamp first so long as it goes there it's ok. Did your tech insert a trim pot on the 6.3 vac line trying to balance it ? Where int he circuit did he insert the trim pot? 

[drew]

I noticed that on the new layout on tube amp doctor the two resistors on the power lamp is soldered to the "bridge" where some of the tiny leads from the transformator also is soldered to.  In my amp, they are soldered to the chassis. (ground)   

It shouldn't make a difference.  The two end terminals of the "bridge" (tag strip) are made from bent metal strips which touch the chassis when the tag strip is screwed down and thus become ground terminals.  The four terminals in the middle are short, straight pieces which are riveted to the phenolic strip and kept insulated from the chassis.  In this amp, they are used as termination points for the unused transformer primary wires (the ones which would be used to wire the amp for Japan, North America, etc. mains voltage). 

[stringbender.dk]

ok, I just thought that maybe the ground was better or something on the tag strip.

mresistor:  no the connection should be the same. but now there are three leads soldered on the tube sockets instead of two.  maybe it's my ocd, but I like the most direct connections.

Well, I havn't heard from the tech today.  maybe he is busy with other projects, or he is busy reading my mails and the links from robrobinette   :)

[stringbender.dk]

This is a short clip, where you can hear the noise. I have no guitar or jackcable in the amp. the click you can hear, is when I pull the standby on and off.
It is before I started rewire the amp...

[shooter]

What I heard was what I call HISS, not HUM.  Hiss typically comes from iffy/sketchy plate resistors, possibly leaky bypass caps (between gain stages).  Have your tech "clip in" a cap, maybe .001 to .01uF across the plate resistor of V3.  if there is no change on V3a, move it to V3b.  This WILL change tone, but you can do tone after fixing.  what volume 1-10 on the guitar drowns out the hiss, or does the hiss increase with guitar volume?

[stringbender.dk]

the amp is down on 0 on the clip

[stringbender.dk]

try turn you speaker up. when they not are turned up, you can't hear the hum.

[sluckey]

Is there more than one person posting under the name "stringbender.dk"?

[shooter]

ok, I was in a noisy coffee shop 1st time.
sounds closer to 100hz than 50, so B+ more so than filament.

things I would quickly check;

the AC ripple on the B+ tap feeding the PI (V3 between the 4.7k and 10K resistors).  I would expect something ~20mVAC or less.

disconnect the NFB, get better or worse
swap the rectifier tube
clip in a .001uf cap across the 100k plate resistor for V2A
remove the 50pF cap across the plates of the PI(V3)
replace the 22nF cap between the tone stack and pin 2 V3

should keep him busy for an hour

[shooter]

the "voice" in post 29 does sound way different than the one in his last post, maybe the tech

[stringbender.dk]

Is there more than one person posting under the name "stringbender.dk"?

not I'm aware of  :)

[stringbender.dk]

the "voice" in post 29 does sound way different than the one in his last post, maybe the tech

What do you mean?

[sluckey]

try turn you speaker up. when they not are turned up, you can't hear the hum.

Stringbender, did you write this? Sounds very different from your other posts in this thread.

[stringbender.dk]

try turn you speaker up. when they not are turned up, you can't hear the hum.

Stringbender, did you write this? Sounds very different from your other posts in this thread.

hmmm... it was me.   I wrote it, because shooter meant it was hiss and not a hum noise my amp does.   And it can sound like a hiss, if you dont turn up for the sound.
it does in my speakers.  I have to turn them up before I can here it in the video   :)    I didn't want to be rude, if you thought that???

[shooter]

if you thought that???

not at all, it was in the wording, not sure if you're using a "translator", your English is better than mine threw out this thread, then when you had me turn up speaker, the English wording seemed way different

[sluckey]

It's not a problem. The wording was just so different from your other post that I thought maybe another person posted that. I was just curious, that's all.

[stringbender.dk]

okay :)   I wrote it in a hurry   

Last night I was reading on this webside about heaters. I fell over this:

 "Make sure that the heater wires on the power tubes stay in phase. Pin two should go to pin two and pin seven should go to pin seven. If you see that they are crossing over from pin 2 on one power tube to pin 7 on the next power tube, you need to unsolder them at one of the power tubes and reverse them. This has the effect of canceling hum in the power amp. You will find them wired wrong all the time."

I didn't wire the heaters over the power tubes in phase. could that be the problem?  could it be that simple?

[sluckey]

I didn't wire the heaters over the power tubes in phase. could that be the problem?  could it be that simple?

I doubt it. I'm not a believer in "proper heater phase" talk, although I do wire my heaters same pin for each tube. My reason is because I'm a symmetrical kinda guy.   

But it's an easy thing to do. Give it a try and see if you can hear a difference. The noise I heard in your video does not sound like heater hum to me.

[Ed_Chambley]

First, what is your tube bias?  Running tubes to hard will make the sound in the video.
The noise sounds constant, not like a resistor. Still could be. The heaters on my original Super Reverb are not phased the same. This is AC current from old design. Here in the States you used to be able to flip a plug over and I never remember it causing a lamp to make more noise.


Go ahead and rework the heater wires and put them in phase, but also keep them away from other wires. Get all your plate wires away from screen wires and components. If you have to place the plate low nearer the chassis and screen wires high and straight down to avoid crossing the plate wires.


Now the place where the tone stack returns to, this is where I would begin.
You should have a preamp ground separate from the power amp ground.

[stringbender.dk]

damn it, I thought I was lucky...... 

Ed chambley:

It got a fixed bias. I don't know what the bias is. right now the amp is at a tech. I think it is the last day today he will try fix it...


How would you make a ground for the preamp and one one for the power amp?      I mean, they are connected via the heaters right?   so how would you separate the ground?

[shooter]

How would you make a ground for the preamp and one one for the power amp?

I marked up your schematic, all the grounds in violet should come together at 1 point only, typically that is the main input jack ground side.  Red grounds all come together at 1 point, typically a mounting bolt for the PT

[bnwitt]

Does your amp have a brass grounding plate into which the control potentiometers are passed through to the chassis?  If so, that is your preamp ground point.  The power section grounds are made elsewhere.  If you have a brass grounding plate and follow the original Fender layout which shows ground points as little triangles then you will have a properly functioning quiet amp.  If you don't have the plate you can substitute it with a ground bus wire running parallel along the back of the pots and connect all of the control and board ground points to it.

The Tube amp Doctor layout shows the preamp ground points as dots.  For instance the 8uf/475V capacitor on the board is shown connected to ground and soldered to the back of the Volume Bright potentiometer which in turn is supposed to be at ground potential by contact to the chassis.  The presence and middle pots also have board ground wires soldered to their backs.  The V1 cathodes (pins 3 and 8) are soldered to the ground lug of the Normal channel input jacks.  etc. etc.  The TAD documentation is very well done showing all grounding points.  If you or your tech just wire the amp exactly like shown using either the Fender or the TAD layouts and barring a faulty or incorrect value component the amp will be fine.

[Leevi]

Try to localize the source of the hum.


Shortcut the grid of the PI to the ground (V3 pin 2) and listen.
If it is still there the reason is probably a ground loop hum.
If it disappears the problem is then between volume and PI, probably some wiring issue,


/Leevi